1. Field of the Invention
This invention relates to a band sawing machine having an endless saw blade extending over two saw pulleys and two blade guides, and more particularly to a mechanism for applying corrective movements to the blade to maintain the blade in a predetermined position.
2. Description of the Prior Art
Blade guides such as are known in the prior art of band sawing machines for the most part serve only the passive role of stabilizing the saw blade during operational use. They generally comprise plates or front faces which are rigidly connected to the machine stand. In some types of band saws, one or both of the blade guides are displaceable as to height in order to allow the machine to accommodate different trunk diameters.
It has been found that, when sawing, the saw blade consistently tends to assume a so-called equilibrium position as a result of the forces of reaction prevailing in the saw blade during sawing. The equilibrium position can vary to some extend, due to different effects, as for instance, a somewhat varying feeding direction of the trunks. Accurate sawing requires that the sawblade occupies a preset, correct position so that the location of a cut can be predicted. In many band sawing machines, the correct position will be a plane which is tangent to the peripheries of both pulleys or which is parallel thereto. The equilibrium position can be displaced to either side of the correct position. It is apparent that the above mentioned lateral displacement has an adverse effect on accuracy of the sawing process.
U.S. Pat. No. 2,914,100, issued to Lindholm, discloses a band sawing machine which attempts to prevent displacement of the saw from determined correct cut position.
In the Lindholm band sawing machine, the saw blade is rotated 90.degree. between the pulleys before the blade passes through a pair of spaced apart blade guides. Each of the blade guides include a slot having a pair of guide surfaces which the sides of the sawblade contact as the blade passes through the slot. The lateral positions of the blade guide are mechanically sensed and used to apply a corrective twist to the blade to return the blade to its correct position.
The blade position control mechanism used with the Lindholm device is inadequate for some purposes for a number of reasons. First, the depth of the cut is limited by the distance between the working portion of the saw blade and the opposite run of the saw blade, a distance which will practically correspond to the diameter of the saw pulleys. Obviously, this is quite insufficient for sawing such as long trunks or other elongated objects.
Second, mechanical position sensing requires a mechanical gauging element contacting the saw blade. The accuracy of measurements made by these structures are inherently degraded by such factors as sawdust, pitch, etc., coating the blades.
Third, the center of twist of the Lindholm device coincides with the teeth of the saw blade. Consequently, corrective twisting produces only an arcurate movement of the rear portion of the saw blade with the lateral position of the teeth remaining practically unchanged. While in some instances it may be sufficient, and even advantageous, to achieve the desired correction by a twisting of the blade in which the position of the row of teeth will remain unchanged, (in other words, where essentially only the inclination of the rest of the blade relative a zero reference plane passing through the row of teeth is changed), it is generally, and in certain specific cases particularly, preferable to have the corrective movement executed as a lateral displacement of at least the row of cutting teeth with the adjacent portion of the blade in a direction opposite to the sensed displacement.
Fourth, twisting the saw blade 90.degree. can seriously reduce the useful life of the blade.
It is clear that the 90.degree. twist of a fast running, continuous saw blade at two locations, and the subsequent continuous varying of the actual angle of twist, will result in a shortened blade life as compared with a blade which is subjected only to those relatively small position changes necessary for displacement compensation. Also a 90.degree. twist precludes the use of the inherent blade tension alone to maintain the blade in contact with the guide surfaces of the guide so that a slotted blade guide must be used. Finally, corrective twisting of the blade which has been pre-twisted 90.degree. is inherently nonsymmetrical and non-linear, i.e. the force required to twist the blade in one direction is not of the same magnitude required to twist the blade in the opposite direction.